1. Field of the Invention
The present invention generally relates to a low noise, highly linear amplifying stage and a signal receiver using the same, which have wide dynamic range and good noise performance by combining active amplifiers and passive attenuators.
2. Description of Related Art
In telecommunication systems, RF (radio frequency) signals or input signals can be distributed to destination devices over a wide area using, for example wireless communication links or wireline communication links. Signal receivers may select between wireless signals received or wireline signals received. In general, wireless signals received are much weaker than wireline signals received. The received signals strength at the receivers may vary over a wide range due to the different characteristics of the wireless links and the wireline links.
TV tuners, which is a kind of signal receivers, need to provide both low noise and highly linear (i.e. wide dynamic range) variable gain. FIGS. 1˜3 show several related arts in signal receivers.
FIG. 1 shows a prior art Radio Frequency (RF) tuner with Automatic Gain Control (AGC) used in conventional signal receivers. System 199 is a Radio Frequency (RF) tuner with Automatic Gain Control (AGC). The system 199 includes: a front-end attenuator 100 that receives an RF input signal, a low-noise amplifier (LNA) 110, a filter 120, a mixer 130, a filter 140, a mixer 150, a digital attenuator 155, and broadband power detectors 160, 170, 180. Power detector 160 is placed after LNA 110, power detector 170 is placed after filter 120, and power detector 180 is placed after mixer 150 and filter 140. Each of power detectors 160, 170, 180 measures the total power in the signal at each respective placement point.
System 199 also includes control block.190 which receives measured power levels from detectors 160, 170, 180 and controls the attenuation by adjusting digital attenuators 100 and/or 150.
However, in FIG. 1, Applicant perceives that the front-end attenuator 100 preceding the LNA 110 constrains the noise figure of the system 199 to be large. That is, Applicant perceives that the noise performance of the system 199 in FIG. 1 is not good enough.
FIG. 2 is a simplified functional block diagram of a prior variable gain amplifier 240. The variable gain amplifier 240 includes a passive attenuator 242 in series with a variable gain stage 244. The amplifier 240 further includes a gain control circuit 246, which controls the attenuation provided by the attenuator 242 and the variable gain provided by the variable gain stage 244, based on feedback signals from other circuits in the signal receiver applying the amplifier 240.
Similarly, Applicant perceives that the passive attenuator 242 preceding the variable gain stage 244 also constrains the noise figure of the variable gain amplifier 240 to be larger than 3 dB. That is, Applicant perceives that the noise performance of the variable gain amplifier 240 in FIG. 2 is not good enough.
FIG. 3 shows another prior AGC amplifier circuit. In FIG. 3, reference numeral 310 represents a signal input terminal, reference numeral 320 represents a fixed-gain amplifier of which the gain does not depend on an AGC voltage, reference numeral 330 represents a variable-gain amplifier (VGA), reference numeral 340 represents a signal output terminal, reference numeral 350 represents an AGC voltage input terminal, reference numeral 360 represents a differential amplifier for level conversion, and reference numeral 370 represents a reference voltage generator circuit. Here, the reference voltage generator circuit 370 supplies a stable reference voltage Vref to the differential amplifier 360. The signal fed in via the terminal 310 is fed to the fixed-gain amplifier 320 and to the variable-gain amplifier 330. The differential amplifier 360 generates AGC voltages Vagc2′ and Vagc2 which vary according to the AGC voltage Vagc.
Applicant perceives that the AGC amplifier circuit in FIG. 3 has limited gain control range because it applies only one VGA. Further, during normal operation, either the active fixed-gain LNA 320 or the active VGA 330 will be active, so Applicant perceives that the linearity performance is also limited.
Therefore, how to solve the above problems is important in RF signal receivers. However, it is difficult to design a very linear amplifying stage capable of amplifying large input signals in a linear fashion which simultaneously possesses continuous or discrete gain control. A further complication is the desire to implement a relatively large dynamic range over which gain control operates. Further, it also needs a RF signal receiver which provides low noise figure, continuous or discrete gain control and wide dynamic range.